1. Field of the Invention
This invention relates in general to supplying power to offshore equipment, and in particular to an apparatus and assembly, and methods associated therewith, for supplying power control and communications to offshore equipment associated with hydrocarbon containing reservoirs.
2. Background of the Invention
The offshore industry has a critical need for an independently deployable, highly reliable, highly available, and low maintenance source of power. This source may be used for powering subsurface and surface pumping, compression needs, other associated fluids conditioning facilities, as well as other power consuming devices. Additionally, supplemental power capabilities are often needed at existing offshore facilities where power demands have grown beyond installed capacity.
Many previously discovered offshore hydrocarbon reservoirs contain producible hydrocarbons, but the producible volumes are insufficient to economically justify the deployment of a dedicated host or stand-alone production facility. In instances where stand-alone development can not be justified the reservoir fluids can sometimes be transported through a pipeline or pipelines either to a neighboring facility to share the infrastructure or to shore to reduce costs and improve the reservoir development economics.
In many of these instances, the distance to the neighboring facility or shore, or tie-back distance, can constitute a major inhibitor to exploitation due to insufficient natural reservoir pressure to adequately support free flowing production. Such tie-back distances can be in excess of fifty (50) or one-hundred (100) miles. Additionally, the water depth associated with a particular reservoir can also be a major inhibitor to exploitation. Pumping, compression and heating stations, either at the reservoir location or at intermediate locations along fluid flowlines or pipelines are typically utilized to promote increased reservoir production.
While the industry has been investigating ways to transport hydrocarbons longer distances and lift production from greater water depths, several problems continue to hamper the reliability and feasibility of exploiting the remote reservoirs. Such a problem can be helped by the use of subsea and surface pumps, compressors, heating, produced fluids conditioning and processing, or a combination thereof, but then arises the problem of providing for large power demands at theses extremely remote, deep, or remote and deep locations.
One particularly difficult situation arises when an isolated and/or ultra deepwater offshore hydrocarbon reservoir could be produced through a host facility with local pressure boosting and/or fluids conditioning, but is where the transmission of necessary power is not technically and/or economically feasible, given the current state of the art.
One method of providing such power is through the use of a steam or other fluid-filled, phase-change cycle based system, such as a Rankin cycle. In general, however, this has not been the method of choice in an offshore environment due to the typical system space, weight, costs, and the initial and operating complexities for an efficient phase-change cycle based system. The selection criteria of space, weight, cost, and complexity tend to be better answered offshore with conventional industrial or aero-devivative fuel fired turbine, and/or reciprocating engine drivers.
The pressure difference between the high and low sides of conventional Rankin cycle is a key factor in the overall cycle efficiency. In general, higher pressure differences yield higher fuel efficiencies. However, in a normal system, attaining high pressure differences adds considerably to the space, weight, complexity and cost of the system.
Typical electric generation systems produce fixed frequency (50 or 60 Hz) alternating current. Where driven equipment (e.g. pumps or compressors) is required to compensate for variations in performance demand, variable speed operation is often the desired option. In the case of electric motors as drivers, electronic variable speed motor controllers are often selected. These electronic variable speed controllers are typically large, costly and highly sensitive to their installation environment.
The output of electronic variable speed controllers is pseudo-sinusoidal electrical current, not the pure sinusoidal current as produced directly by an electrical generation facility. Transmitting pseudo-sinusoidal electrical current through power lines results in harmonic feedback system instability and inefficiency that increase with transmission distance. The available technology for large load distribution (especially variable frequency energy as used in variable speed motors) severely limits the maximum technical or economic transmission distance and/or water depth.
Remote and/or unattended power generation schemes including “buoy” supported systems, have been deployed using moderately reliable, maintenance intensive diesel engine driven engines. While the systems are functional, constant operator intervention and maintenance operations have made it difficult to maintain the desired system availability in a primarily unmanned remote offshore environment. Fuel supply logistics and quality are typical problem areas of conventional diesel fueled engines.
Gas turbine technology has also been considered for “buoy” based or remote power generation. However, gas turbines present similar challenges to availability, fuel quality, and frequent visits by maintenance personnel to those encountered with diesel driven systems.